This invention generally pertains to structures and methods exploiting silicon processing technology for accomplishing both arcuate and straight segment shapes of path trajectory for an optical fiber within a groove across a surface of a supporting substrate. It particularly applies when the surface region of side-polish on the fiber is generally co-planar with the silicon surface containing the groove and the arcuate path within the groove lies in a plane that is parallel to the normal to the silicon surface. It applies to single-mode optical fibers as well as optical fibers that are not single-mode.
An objective of this invention is to improve the yield and lower the cost of manufacture for precision side-polished fiber optic components.
There are no prior art methods, structures, apparatuses, or devices published or on the market for utilizing precision silicon processing technology to achieve ultra-precise alignments of interacting side-polished fiber optics as when joining two side-polished fibers face to face against their side-polished areas. What is known in the related prior art deals with implementation of single fibers that are side-polished to implement two-port photonic functions requiring no side-by-side critical alignment to other fibers. This prior art is taught in U.S. Pat. No. 5,809,188 xe2x80x9cTunable optical filter or reflectorxe2x80x9d and U.S. Pat. No. 5,781,675 xe2x80x9cMethod for preparing fiber-optic polarizerxe2x80x9d, both by Tseng.
Tseng""s patents teach the use of V-grooves etched in a 100 surface of a silicon crystal substrate and having a continuously varying groove depth. Note that xe2x80x9c100xe2x80x9d refers to crystal orientation as defined by Miller indices as generally known in the field of crystallography and silicon processing. Tseng""s patents teach how such a continuous arcuate groove achieves both a) an arcuate path for guiding the fiber along an arcuate path trajectory and b) precise control of the remaining side-wall thickness left along a controlled length of the arcuate, side-polished fiber that is bound within the groove.
Tseng""s patents teach methods to achieve superior precision in groove dimensions and consequent control of remaining, polished, sidewall thickness to a fiber held within a silicon V-groove. Using this technique, the cross-sectional shape of a groove running parallel to the 110 direction is that of a xe2x80x9cV-shapexe2x80x9d, wherein the side-walls of the xe2x80x9cV-shapexe2x80x9d are 111 planes. This technique works well because the 111 etch direction has a significantly slower etch rate than any other direction. A continuous change in groove depth is used to achieve an arcuate path with a long radius of curvature for holding a fiber and thereby guiding the fiber itself into an arcuate path trajectory.
But a disadvantage exists with attempting to achieve a precise groove depth with Tseng""s technique. If the groove was of constant depth (and width), the side-walls would indeed be 111 planes, and since they are the slowest to etch, the cross-section would be precisely determined by width of the etch mask. However, Tseng teaches the use of a smoothly curved groove depth (and width) thereby causing the side-walls to be determined by a very large set of 111 planes intersecting the groove. This makes the groove depth at the shallowest point harder to define by etch-rate and etch-time, since the etchant can attack the groove-forming substrate from directions other than just the 111 direction.
Another shortcoming of the above cited Tseng inventions is that the length of the side-polished area is strongly a function of the radius of curvature of the arcuate path of the fiber. In order to achieve a shorter length to a side-polish on a fiber, it then becomes necessary to use a shorter radius of curvature. The current invention overcomes this disadvantage by utilizing a straight middle portion of path trajectory. At the ends of the straight portion, the current invention uses either short-radius, arcuate, portions of path length, or portions of groove that are deeper in the substrate. This can make the side-polish length more a function of the length of the straight portion and less a function of the particular fiber-path radius or radii used within the end portions. This also allows two such fibers to be joined face-to-face against their side-polished areas to create optical couplers wherein the interaction length of coupling can be selected or adjusted. With the current invention, this adjustment can be accomplished more precisely either by design specification of the length of the straight middle portion of the groove or by shifting one fiber relative to the other along the direction parallel to the groove axes. This is because the arcuate end portions can be made relatively short compared to a path determined by a single radius of curvature.
No prior art teaches methods, structures, apparatuses, or devices for achieving an arcuate fiber trajectory by using a few steps in mask pattern width or by few incremental steps between portions of the groove wherein each portion has a constant depth. And no prior art teaches use of either arcuate or stepped grooves to achieve an arcuate fiber trajectory having a long straight portion in the middle. And no prior art teaches use of a groove that has a midportion that is of constant width and depth joined with end portions that increase in depth with transitions parallel to crystal planes.
Additional prior art on positioning of fiber optics on substrates is found in the technology of Microelectronic Mechanical Systems (MEMS). One reference to such technology is that of xe2x80x9cMEMS Packaging for Micro Mirror Switchesxe2x80x9d, by L. S. Huang, S. S. Lee, E. Motamedi, M. C. Wu, and C. J. Kim, Proc. 48th Electronic Components and Technology Conference, Seattle, Wash., May 1998, pp. 592-597. But this prior art does not teach the use of arcuate groove shapes or arcuate fiber trajectories, since a single V-groove of constant width and depth cannot alone shape an arcuate path for an optical fiber.
Another aspect of using a groove to guide the path shape of a fiber is that continuous contact between a fiber and a groove with either a straight or arcuate trajectory is susceptible to contamination in the form of a particulate or a film. Mechanical interference with such contamination can alter the accuracy and/or precision of the trajectory (i.e. shape) of the fiber. For example, a contaminating particle or film positioned between a surface of the groove and the surface of the fiber it guides can perturb the position of the fiber from precisely following the contour of the groove.
What is needed are structures and methods that can utilize a combination of groove features, using length-wise portions that each have constant width and constant depth to better exploit the dimensional controls achievable with photolithography and etching to guide an optical fiber along a straight or arcuate trajectory. The current invention provides these structures and methods, provides better determination of interaction length for couplers (and other 4-port fiber optic components) and reduces susceptibility to fiber mis-alignment caused by particulate or film contamination on either the groove surface(s) or on the surface of the fiber.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods and structures (or apparatuses or devices) and combinations particularly pointed out in the appended claims.
The objects of the invention are principally five-fold: a) to provide methods and structures to accomplish a precise and accurate fiber trajectory or path within a groove in the surface of a crystal substrate for creating a side-polished optical fiber wherein a portion of the fiber path is straight, b) to cause the length of side-wall polish for an optical fiber in such a groove to be a strong function of the length of the straight portion and less dependent on the length(s) or radius (or radii) of arcuate portions, c) to accomplish more precise and accurate control of side-wall thickness in the cladding of an optical fiber, d) to provide for lengthening or shortening a polished side-wall while simultaneously reducing the length(s) of arcuate portion(s), and e) to reduce the probability of a contaminating particle or film from perturbing the trajectory of a fiber.
Within this specification, and applied to this invention, the word xe2x80x9cstructuresxe2x80x9d is used to include xe2x80x9capparatusesxe2x80x9d or xe2x80x9cdevicesxe2x80x9d. Also, xe2x80x9ctrajectoryxe2x80x9d is used to refer to a fiber or groove path extending over the length of a groove.
These and other objects of the invention are provided by a novel use of stepped V-grooves in a crystalline substrate. A central portion (or one or both end portions) of the fiber length along the groove can be straight. One or both ends of a first fiber, and of a groove containing the fiber, can curve farther into the substrate to provide mechanical clearance for the end portions of the fiber to be protected from a side polishing tool at the surface of the substrate. This also provides later for clearance from unpolished portions of a mating side-polished second fiber.
The curvature required of side-polished fibers is slight (radii of curvature designed anywhere from approximately 10 cm to 2000 cm). Therefore, a fiber can be physically routed by a groove structure composed of a sequence of generally contiguous segments each specified to be of a constant depth (and width if the cross-section is to be V-shaped). Moving away from the portion of the groove that has the smallest depth, the depths of the other groove segments can increase gradually. As an optical fiber is reasonably stiff and cannot be bent around sharp corners, the trajectory a fiber takes when placed within such a segmented and stepped groove is determined by contact points along the length of the shallowest portion and the outer end portions of each successively deeper adjacent segment.
Between adjacent segments, the fiber can be free-floating, unless or until such time as it is connected to the groove side-walls via a thickness of bonding material. There are two advantages of having portions of the fiber be free-floating at least during the initial assembly of the fiber into the groove. One advantage is that there is less of the fiber length susceptible to position displacements which could otherwise be caused by contamination particles or films coming between the fiber and a groove surface. The second advantage is that, should the fiber need to be removed from the substrate and groove, it is easier to etch, dissolve, or otherwise remove a bonding material from between the fiber and the groove if there is a larger access path to the fiber surface portions that face the groove surfaces.
The preferred implementation of this invention utilizes a groove geometry comprised almost entirely of 111 crystal planes. This preferred implementation uses a sequence of adjacent length portions each having constant width and depth but stepping between magnitudes from portion to portion. Even the short transition between any two of these adjacent portions can be comprised of a 111 crystal plane surface. With a significant length to the shallowest portion of the groove, the remaining side-wall thickness on the clad of an optical fiber after polishing the fiber down to the surface of the substrate, can therefore be very precisely and accurately controlled.
The reader will readily appreciate the novel use of a segmented groove geometry to better exploit the etch-stop properties of 111 crystal planes in determining a groove geometry for precise and accurate control of remaining side-wall thickness in the cladding of an optical fiber side-polished within a crystalline substrate of silicon or even other III-V materials.